This invention pertains generally to microwave oscillators and in particular to a low noise microwave oscillator.
It is known in the art that microwave oscillators having low noise for low noise applications are necessary for many advanced electronics systems. In a microwave oscillator, frequency modulation (FM) noise is a problem due to the noise characteristics of the components in the oscillator. It is advantageous to reduce the FM noise characteristics of a microwave oscillator. One technique known for reducing noise in a microwave oscillator is described in U.S. Pat. No. 5,032,800, issued Jul. 16, 1991, entitled "Tunable Oscillator with Noise Degeneration," (which patent is assigned to the same assignee as this application). In said patent an oscillator is described including an amplifier having an input and an output and a feedback circuit disposed between the input and the output of the amplifier. The feedback circuit includes a yttrium-iron-garnet (YIG) filter and a voltage-controlled phase shifter disposed in series with the YIG filter. The oscillator further includes a discriminator circuit responsive to signals from the voltage-controlled phase shifter and the YIG filter, for providing a control signal to a control port of the voltage-controlled phase shifter for degenerating close-to carrier noise within the oscillator caused by up-converted low frequency noise arising in the amplifier. With this particular arrangement, using a control port of the voltage-controlled phase shifter as the tuning port of the oscillator provides an oscillator having predictable oscillator modulation sensitivity from oscillator to oscillator and a modulation sensitivity which is independent of the signal level in the oscillator feedback loop until very high signal levels are reached. Additionally, a frequency tuning capability is provided for oscillators using one port amplifier devices such as Gunn diode amplifiers and IMPATT diode amplifiers.
An alternative technique for reducing noise when using a gallium arsenide FET as an active element is a single resonator microwave oscillator as described in U.S. Pat. No. 4,555,678, issued Nov. 26, 1985, entitled "Microwave Oscillator" (which patent is assigned to the same assignee as this application). In that patent, it is described that a gate bias port of a FET amplifier can be used as the tuning port of the oscillator since the gate to source capacitance of the FET is dependent on gate to source voltage. Thus a signal applied between the gate and source terminals changes the value of the gate to source capacitance which in turn modulates the phase of the signal amplified by the FET. The dispersive phase response of the resonator in the feedback circuit converts the phase modulation of the amplified signal to a frequency modulated signal at the output of the oscillator. Thus the frequency of the oscillator is modulated by the signal applied to the gate bias port serving as the tuning port of the oscillator.
Although each of the above described techniques are useful in many applications, microwave oscillators often require frequency agility and low FM noise but at a higher power level and with less cost. For example, in an application wherein the oscillator is operating at high power near the saturation limit of a resonator including a YIG filter, the FM noise level of the oscillator is increased severely. Also, FM noise can be induced by eddy currents within the YIG filter, particularly in the RF structure. Although FM noise can be reduced by a technique as described in U.S. Pat. No. 4,758,800, issued Jul. 19, 1988, entitled "Low Noise Magnetically Tuned Resonant Circuit" (which patent is assigned to the same assignee as this application), such a technique adds to the complexity of the circuitry.